Variable Throat Venturi Flow Meter

ABSTRACT

An apparatus for performing a well test operation includes a closed chamber testing system having a downhole flow control device. In some embodiments, a method of performing a well test operation includes performing a well test operation with a closed chamber testing system having a downhole flow control device and estimating the reservoir properties based on pressure and flow data measured during the well test operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/808,203, filed Sep. 20, 2010, which is herein incorporatedby reference.

BACKGROUND

1. Field of the Invention

The invention relates generally to an apparatus for measuring flow ratesin a fluid such as in oil and gas wells.

2. Description of the Related Art

Venturi flow meters are commonly used to measure flow rates inmultiphase fluids in a well bore, such as oil, gas, and water producedfrom hydrocarbon underground reservoirs. An example of such a flow meteris disclosed in U.S. Pat. No. 4,856,344. The basic premise of a Venturiflow meter is to introduce a restriction in the flow path. Therestriction is commonly referred to as the “throat.” Fluid pressure ismeasured in the throat and at a location upstream of that restriction.Because the flow area in the throat is smaller than the upstreamportion, the velocity of the fluid increases, which results in acorresponding decrease in pressure in the throat, according to the wellknown Bernoulli principle according to which the total energy in asteadily flowing fluid system is a constant along a flow path. Bycomparing the two pressure readings and the respective cross-sectionalareas, a flow rate can be determined using the relationship betweenfluid velocity and pressure.

FIG. 1 shows a basic layout of a typical Venturi flow meter 100 disposedwithin a pipe 101 that can be used in a well bore. The upward flowdirection of the fluid through the flow meter is shown by arrow F. Theprimary portions are an upstream section 120 and a throat section 110. Aconvergent tapered section 115 connects the upstream section 120 and thethroat section 110. The convergent tapered section 115 provides a smoothreduction in diameter between the larger upstream section 120 and thesmaller throat section 110 in order to minimize turbulence in the flowmeter. Downstream of the throat section 110, a divergent tapered section130 provides a smooth transition to the larger downstream section 140,which reduces the fluid velocity and minimizes turbulence and associatedpressure losses. A pressure gauge P3 measures the fluid pressure in thethroat section 110 where the diameter is D2. Another pressure gauge P2measures the fluid pressure in the upstream section 120 at a pointupstream of the convergent tapered section 115 at a point where the pipediameter is D1. The ratio of the diameter D2 over the diameter D1 isknown as the Beta ratio of the Venturi flow meter and is generally about0.5. Taking into account the Beta ratio, a comparison between the twopressure gauges P2 and P3 provides a differential pressure, which allowsfor the determination of the flow rate of the fluid based on well knownequations.

In addition to flow rate measurements, a typical Venturi flow meter mayfurther include a gradiomanometer to determine the density of amultiphase fluid. To determine the average density of the multiphasefluid, a differential pressure is taken between two pressure gauges P1and P2 located in the upstream section 120 of the flow meter 100 andseparated by a distance H1. The differential pressure correlates withthe average density of the multiphase fluid between the two pressuregauges P1 and P2.

In a typical oil well, inside the wellbore where the hydrocarbon fluidis flowing, the fluid flow rate may vary significantly, especiallyduring transient periods of testing and production. In particular, whencarrying out drill stem testing (DST), which is used to evaluate theproduction potential of hydrocarbon reservoirs, the fluid flow rate maybe lower than expected or vary widely. Thus, the pressure differentialbetween the throat section and the upstream section may be too small sothat accurate flow rate measurement results may not be obtained. Inorder to increase the pressure differential and thus to be able toaccurately measure small flow rates, the cross- sectional area of thethroat section is made to be very small. However, if the cross-sectionalarea of the throat section is very small, various downhole tools willnot pass through the Venturi flow meter. For example, production loggingtools and firing guns deployed by wireline may have a maximum diameterof over 5 cm (2 in.), making it impossible for those tools to passthrough the small throat section. This is one of the reasons whyconventional flow meters have not been used successfully downhole in awellbore, and typically, the flow rate has to be measured at thesurface.

SUMMARY

The present invention relates to a variable throat Venturi flow meterthat overcomes the cited and other limitations of the prior art. Thevariable throat Venturi flow meter according to the invention provides anew way of measuring the fluid velocity inside the wellbore. Inaddition, by varying the throat cross-section in accordance withembodiments of the present invention, the throat cross-section can beadjusted to accommodate the passing of downhole tools and to accommodatewidely variable flow rates inside the wellbore.

In a first aspect, the invention relates to a flow meter comprising anupstream pipe section having an upstream cross-sectional area, a throatsection having a throat cross-sectional area that is smaller than theupstream cross-sectional area, an upstream port disposed in the upstreampipe and in communication with a first pressure sensor, a throat portdisposed in the throat and in communication with a second pressuresensor, and means for varying the throat cross-sectional area.

In a second aspect, the invention relates to a method of measuring aflow rate in a well bore. The method includes deploying a variablethroat Venturi flow meter in the well bore. The variable throat Venturiflow meter includes a throat section of which the throat cross-sectionalarea is variable between a first throat cross-sectional area and asecond throat cross-sectional area, wherein the first throatcross-sectional area is smaller than the second throat cross-sectionalarea. A flow rate is determined using pressure measurements provided bythe variable throat Venturi flow meter using the first throatcross-sectional area. The throat cross-sectional area is temporarilyvaried to have the second throat cross-sectional area to allow adownhole tool to pass through the throat section.

Other aspects, characteristics, and advantages of the invention will beapparent from the following detailed description and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art Venturi flow meter.

FIGS. 2-4 show a variable throat Venturi flow meter in accordance with afirst embodiment of the present invention.

FIGS. 5 and 6 show a variable throat Venturi flow meter in accordancewith a second embodiment of the present invention.

FIGS. 7 a and 7 b show a variable throat Venturi flow meter inaccordance with a third embodiment of the present invention.

FIGS. 8 a and 8 b show a variable throat Venturi flow meter inaccordance with a fourth embodiment of the present invention.

FIG. 9 shows a downhole flow rate testing system in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

In a first aspect, the present invention provides a variable throatVenturi flow meter comprising an upstream section with a substantiallyconstant cross-sectional area and a throat section with a variablecross-sectional area.

The cross-sectional area of the throat section of the variable throatVenturi flow meter according to the invention may vary and can bereadily adjusted to accommodate the expected flow rate to be measured.For example, if the flow rate is low, a smaller throat cross-sectionalarea is needed to provide accurate flow measurements. As the flow ratemay increase, the cross-sectional area of the variable throat Venturiflow meter may also increase.

Turning to FIGS. 2-4, a variable throat Venturi flow meter 200 inaccordance with a first embodiment of the invention is shown. FIG. 2 isa cross-section of the flow meter. FIGS. 3 and 4 are perspective viewsof the flow meter (partial view). The variable throat Venturi flow meter200 comprises an upstream section 220 and a throat section 210. In thisembodiment, a convergent tapered section 215 connects the upstreamsection 220 and the throat section 210. The throat section 210 includesa pair of rotatable inserts 211 and 212. The rotating inserts 211 and212 are generally cylindrical in shape, with their principal axes beingperpendicular to the principal axis of the flow meter, i.e., the flowdirection F. In order to vary the cross-sectional area of the throatsection 210, each of the rotatable inserts 211 and 212 includes acut-out portion 222 and 224, respectively. The cut-out portions 222 and224 may be of a half-cylindrical form such as that the principal axes ofthe cut-out portions 222 and 224 are orthogonal to the principal axes ofthe rotatable inserts 211 and 212, as shown in FIGS. 3 and 4.

In FIGS. 2 and 3, the rotatable inserts 211 and 212 are in the “closed”position. In FIG. 4, the rotatable inserts 211 and 212 are in the “open”position. In the closed position, the throat cross-sectional area isreduced to area A3. In the particular embodiment shown in FIGS. 2-4, thecut-outs 222 and 224 are formed to substantially match the diameter D2of the throat section 210 such that a substantially cylindrical throatsection is provided when the rotatable inserts 211 and 212 are in theopen position. The actuation of the rotatable inserts 211 and 212 may beperformed using a variety of mechanisms such as using spring actuators(not shown) or an electric motor 205, or any other actuation mechanismknown in the art. The actuation may be initiated, for example, inresponse to pressure or electrical signals interpreted by a controllersystem 217 integrated in the flow meter 200 and/or by lowering adownhole tool into the flow meter 200 until it reaches the rotatableinserts 211 and 212, which may respond by rotating into the openposition. In the closed position, during flow periods, the throatcross-sectional area may also be adjusted continuously by a controllersystem linked to a continuous real-time differential pressuremeasurement.

In the embodiment shown in FIGS. 2-4, the open position provides a fullthroat opening. In downhole use, the flow meter 200 may be deployed inthe open position. Should the flow rate decrease, the rotatable inserts211 and 212 may be rotated partially towards or completely to the closedposition, thus reducing the throat cross-sectional area, therebyproviding an increase in differential pressure between pressure gaugesP2 and P3. In the closed or partially closed position, the throatcross-section may be non-circular, which is the case in the presentembodiment. Although this somewhat complicates the Beta ratio by notbeing a comparison of diameters, an effective Beta ratio can be providedby comparing the effective throat cross-sectional area with thecross-sectional area of the upstream section 220. Experimental data maybe needed to compensate for irregular flow through the non-circularthroat cross-section when the rotatable inserts 211 and 212 are in theclosed or partially closed positions. Preferably, the edges of therotatable inserts 211 and 212 should be curved or rounded to reduce oreliminate turbulence and to improve accuracy in the pressure readings.

Turning now to FIGS. 5 and 6, a second embodiment of the variable throatVenturi flow meter in accordance with the present invention is shown. Aswith the previous embodiment, the throat cross-section may be varied.The throat cross-section is varied using revolving inserts 340 and 341disposed in a rotatable sleeve 330. The revolving inserts 340 and 341include pivots 350 at an upper location. An angled ramp 310 is providedon the back side of the revolving inserts 340 and 341 and/or on theinside of the sleeve 330. Because of the angled ramp 310, rotation ofthe sleeve 330 causes the revolving inserts 340 and 341 to bealternately forced inward (i.e., closed position, FIG. 6) or allowed toopen outward (i.e., open position, FIG. 5). In the embodiment shown inFIGS. 5 and 6, the revolving inserts 340 and 341 include cut-outs 320,which may provide a substantially circular throat cross-section in theclosed position when the revolving inserts 340 and 341 are broughtinwardly together. Because of the irregular throat cross-section andvoid behind the revolving inserts 340 and 341, this embodiment may proveunsuitable to provide accurate flow measurements when the revolvinginserts 340 and 341 are in the open position. However, the open positionallows for the deployment of downhole tools through the flow meter thatare too large to pass through the flow meter when the revolving inserts340 and 341 are in the closed position. Accordingly, the embodimentshown in FIGS. 5 and 6 provides accurate flow rate measurements for lowflow rates in the closed position, while still allowing for downholetools to be deployed past the flow meter.

The revolving inserts 340 and 341 may be actuated in response to theforce of the downhole tool or command signals communicated to acontroller system (not shown). In one embodiment, the sleeve 330 isspring loaded so as to be biased towards the closed position to provideflow rate measurements. The force of the springs (not shown) may beselected to be sufficiently stiff to be stable during flow ratemeasurements, but weak enough to allow the weight of various downholetools to force the revolving inserts 340 and 341 into the open position.Upon removal of the downhole tool, the revolving inserts 340 and 341would automatically return to the closed position to allow for continuedflow rate measurements. Alternatively, an actuator (not shown) mayrotate the sleeve 330 between the open and closed positions.

In FIGS. 7 a and 7 b, a third embodiment of the variable throat Venturiflow meter in accordance with the present invention is shown. In thisembodiment, the throat cross-section is varied by providing springblades 701 in the throat section. The spring blades 701 are inwardlybiased to restrict the flow through the throat section, which providesthe differential pressure necessary for flow rate measurements. Thespring blades 701 provide a simple variable throat section bysimultaneously providing the convergent and divergent tapered sectionsof the variable throat Venturi flow meter. The spring blades 701 areselected to be flexible enough to be forced outwardly by variousdownhole tools, but stiff enough to return to the inward, closedposition upon removal of the downhole tool. Dead volumes 702 between thebackside of the spring blades 701 and a throat body 710 may be filledwith highly viscous fluids or materials under pressure to avoidinterfering with the multiphase fluid passing through the flow meter.For example, a highly viscous fluid may be enclosed in two recipients(not shown). The recipients should be of an elastic, non-corrosivematerial, and they may respectively be attached to the throat body 710at the backside of each spring blade 701. Another example could be opencells with foams or gels. The spring blades 701 thus respond to axialforces. The axial forces are exercised by downhole tools passing throughthe throat section and forcing outwardly the spring blades 701 and bythe viscous fluid driving back the spring blades 701.

FIGS. 8 a and 8 b show a preferred embodiment of the invention thatcombines a variable throat Venturi flow meter according to the thirdembodiment with a density/viscosity sensor. The density/viscosity sensor810 is positioned in the throat section 210, thus providing a directsingle- or multi-phase fluid density measurement at the Venturi throat.This density/viscosity sensor design may be based on micro resonatortechnology, thus being small enough to be inserted into the throatsection 210 without affecting the flow rate measurements.

In the embodiments disclosed herein, the pressure difference may also bemeasured using a differential pressure sensor disposed in the throatsection, instead of using the pressure gauges P2 and P3 in the upstreamsection and the throat section, respectively.

In a second aspect, the invention provides a method of measuring a flowrate in a well bore. The method comprises lowering and installing avariable throat Venturi flow meter according to embodiments of theinvention into the wellbore. The variable throat section of the flowmeter has a throat cross-sectional area that is variable between a firstthroat cross-sectional area and a second throat cross-sectional area,wherein the first throat cross-sectional area is smaller than the secondthroat cross-sectional area. The flow rate is then determined usingpressure measurements provided by the variable throat Venturi flow meterwhen the throat section has the first throat cross-sectional area. Bytemporarily varying the throat cross-sectional area to have the secondthroat cross-sectional area, a downhole tool is able to pass through thethroat section. Once the downhole tool has been withdrawn, the throatcross-sectional area can be re-adjusted to the first throatcross-sectional area.

The throat cross-sectional area of the flow meter may be adjusted insuch a way that the differential pressure is kept constant or onlyslightly varying. Thus, a discharge coefficient and Reynolds numbervariation can be minimized. The flow rate of a single phase liquidacross the flow meter may then be determined by using the followingfluid equation:

$\begin{matrix}{{Q = {{CA}\; ɛ\sqrt{\frac{2\; \Delta \; p}{\rho}}}},} & (1)\end{matrix}$

where Q is the flow rate, C is the discharge coefficient which is afunction of the Reynolds number, A is the throat cross-sectional area, εis expansibility factor, Δp is a pressure difference across the venturi,and ρ is the fluid density.

To calculate Q using equation (1), the pressure difference Δp ismeasured using two pressure gauges disposed in the throat section and inthe upstream section of the flow meter. For a single-phase liquid flowthe expansibility factor ε can be approximated to 1. Since the dischargecoefficient C cannot be calculated with certainty, it is generallydetermined through experimentation. It may vary over a wide range as afunction of the Reynolds number. Adjusting the throat cross-sectionalarea A as a function of the pressure difference Δp will minimize theReynolds number variation and therefore minimize the error anduncertainty on the discharge coefficient C. Thus, the flow rate may alsobe determined with greater certainty.

Referring now to FIG. 9, a wellbore comprising a downhole flow testingsystem 900 is schematically shown. The downhole flow testing system 900comprises the variable throat Venturi flow meter 200 according to theinvention and a command module 901. The controller system (217, FIG. 2)of the variable throat Venturi flow meter 200 is configured to receiveopen and close signals from the command module 901 in order to vary thecross-sectional area of the throat section of the flow meter.

Embodiments of the present invention may provide one or more of thefollowing advantages. The ability to vary the throat cross-sectionalarea in situ avoids the need to remove the flow meter from the wellboreto replace the flow meter. This allows for the throat to be variedaccording to flow rates experienced in the wellbore. Further, low flowrates can be measured by reducing the throat cross-sectional areawithout the disadvantage of then being unable to accommodate downholetools because the reduction is easily reversible in situ.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A variable throat Venturi flow meter comprising: an upstream pipesection having a substantially constant upstream cross-sectional area; athroat section having a variable throat cross-sectional area that issmaller than the upstream cross-sectional area; and a plurality ofsection-varying elements adapted to temporarily vary the throatcross-sectional area to allow a downhole tool to pass through the throatsection.
 2. The variable throat Venturi flow meter of claim 1, furthercomprising: a tapered section connecting the upstream pipe section andthe throat section.
 3. The variable throat Venturi flow meter of claim1, further comprising: an upstream port disposed in the upstream pipesection and in communication with a first pressure sensor; and a throatport disposed in the throat section and in communication with a secondpressure sensor.
 4. The variable throat Venturi flow meter of claim 1,further comprising: a differential pressure sensor disposed in thethroat section.
 5. The variable throat Venturi flow meter of claim 1,wherein the plurality of section-varying elements comprise spring bladesarranged to vary the throat cross-sectional area in response to axialforce.
 6. The variable throat Venturi flow meter of claim 5, furthercomprising: a viscous fluid or material disposed behind the plurality ofspring blades.
 7. The variable throat Venturi flow meter of claim 5,further comprising: a density/viscosity sensor disposed in the throatsection.
 8. The variable throat Venturi flow meter of claim 2, whereinthe plurality of section-varying elements comprise at least tworotatable inserts configured to vary the throat cross-sectional area byrotating between an open position and a closed position.
 9. The variablethroat Venturi flow meter of claim 2, wherein the plurality ofsection-varying elements comprise at least two revolving insertsdisposed in a rotatable sleeve.
 10. The variable throat Venturi flowmeter of claim 1, wherein the throat cross-sectional area varies inresponse to a command signal.
 11. The variable throat Venturi flow meterof claim 10, wherein the command signal is one of a pressure signal andan electrical signal.
 12. A downhole flow testing system comprising: acommand module; and a variable throat Venturi flow meter disposed in awellbore, the variable throat Venturi meter comprising: an upstream pipesection having a substantially constant upstream cross-sectional area; athroat section having a variable throat cross-sectional area that issmaller than the upstream cross-sectional area; a plurality ofsection-varying elements adapted to temporarily vary the throatcross-sectional area to allow a downhole tool to pass through the throatsection; and a controller system configured to receive open and closesignals from the command module.